This invention relates to improvements in developing apparatus for electrostatic printing machines, and more particularly, to improvements in a magnetic brush developing apparatus for accomplishing highly efficient, fast speed development of an electrostatically formed image.
In the practice of xerography as described in U.S. Pat. No. 2,297,691 to Chester F. Carlson, a xerographic surface comprising a layer of photoconductive insulating material affixed to a conductive backing is used to support electrostatic images. In the usual method of carrying out the process, the xerographic plate is electrostatically charged uniformly over its surface and then exposed to a light pattern of the image being reproduced to thereby discharge the charge in the areas where light strikes the layer. The undischarged areas of the layer thus form an electrostatic charge pattern in conformity with the configuration of the original pattern.
The latent electrostatic image may then be developed by contacting it with a finely divided electrostatically attractable material, such as a resinous powder. The powder is held in the image areas by the electrostatic fields on the layer. Where the field is greatest, the greatest amount of material is deposited; and where the field is least, little or no material is deposited. Thus, a powder image is produced in conformity with the image of the copy being reproduced. The powder is subsequently transferred to a sheet of paper or other transfer member and suitably affixed to thereby form a permanent print.
The latest concept for copiers utilizes high speed flash exposure of a document and the arrangement of a moving photoconductive material in the form of an endless belt which is continuously charged. Additionally, such copiers are provided with a developing system which supplies toner particles in relatively large quantities for solid area coverage, such as a magnetic brush developing apparatus.
The electrostatically attractable developing material commonly used in magnetic brush developing apparatus comprises a pigmented resinous powder referred to here as a "toner" and a "carrier" of larger granular beads formed with steel cores coated with a material removed in the triboelectric series from the toner so that a triboelectric charge is generated between the toner powder and the granular carrier. The magnetizable carrier also provides mechanical control for the formation of brush bristles by virtue of magnetic fields so that the toner can be readily handled and brought into contact with the exposed xerographic surface. The toner is then attracted to the electrostatic latent image from the carrier bristles to produce a visible powder image on an insulating surface of the photoconductive material. Generally, in an endless belt printing machine configuration which employs a plurality of magnetic brushes, the brushes are arranged for developing purposes with a run of the belt in the planar orientation.
In copending application Ser. No. 97,856, now U.S. Pat. No. 3,724,422, issued on Apr. 3, 1973 to Salvatore Latone and Michael R. Stanley, entitled "Magnetic Brush Developing Apparatus", and assigned to the same assignee, there is described a magnetic brush developing assembly comprised of a plurality of magnetic brushes having their axes arranged in parallel and in a plane generally parallel to the inclined plane of the photoreceptor belt at the zone of development therefor. The brushes are arranged so that the developing material in bristle form is moved in the inclined plane, whereupon, after moving past the last brush in the string of brushes, the material is returned for reuse to the sump at the beginning end of the string by gravity forces.
In order to improve homogeneity of the developing mixture and prevent toner particle depletion in any zone of the apparatus, a cross-mixing baffle assembly is provided, such as described in copending application Ser. No. 97,688, now U.S. Pat. No. 3,697,050, issued on Oct. 10, 1972 to Michael R. Stanley, entitled "Cross-Mixing Baffle", and assigned to the same assignee. In the lower portion of the sump of the developing apparatus, a paddle wheel impeller is provided to transport developing material towards the area adjacent the first magnetic brush of the magnetic brush developing apparatus.
In the magnetic brush developing apparatus heretofore used, arcing at various locations occurred which constituted a potential hazardous situation, as well as to effect the charge on the photoconductive surface thereby effecting the quality of the resulting image on a transfer member. Additionally, the developing material in the sump zone has suffered from lack of homogeneity, especially when a plurality of copies are being produced having large selected portions of solid area coverage. The relative positioning of the magnets within the rollers of the magnetic brush developing apparatus has resulted in the piling-up of developing material in undesirable locations within the housing with concomitant excessive clean-up requirements. Still further, caking of developing materials has occured about the trimming bar configurations heretofore employed.